1. Field of the Invention
The present invention relates to a wire bonding apparatus and more particularly to a back tension device used in a wire bonding apparatus so as to apply back tension to the bonding wire.
2. Prior Art
In a wire bonding apparatus, back tension is applied to the wire by a back tension device so that the wire is prevented from sagging and is smoothly supplied to a bonding tool. One type of back tension device is described in, for example, Japanese Patent Application Publication (Kokoku) Nos. S63-52778, H1-48656 or H8-8271. This structure is described in FIG. 2.
In FIG. 2, a bonding wire (merely called "wire") 1 which is wound on a spool (not shown) is extended to the tip end of a bonding tool 3 via a wire damper 2. Air 5 is blown onto the wire 1 between the spool and the wire damper 2 by an air-blowing means 4 which is, for instance, an air nozzle so that back tension is applied to the wire 1. When air 5 is not blown, the wire I is put in a slack state as shown by the two-dot chain line. In FIG. 2, the reference numeral 6 is an air guide which is attached to the air-blowing means 4, and 7 is a wire guide.
Though not disclosed in the prior art noted above, the air-blowing means 4 is connected to a compressor 11 via an electromagnetic valve 8, an air flow meter 9 and a regulator 10. Accordingly, back tension to the wire 1 is applied while the electromagnetic valve 8 is open by keeping the valve 8 ON, or back tension is applied to the wire by switching the electromagnetic valve 8 ON when wire bonding is restarted after the valve 8 is switched OFF following a fixed pause period in wire bonding.
In the prior art described above, when the electromagnetic valve 8 is switched ON to supply air 5 to the air-blowing means 4 (from a state in which the electromagnetic valve 8 is switched OFF so that the supply of air 5 to the air-blowing means 4 is stopped), air 5 is fed with a pressure involving abrupt fluctuation; accordingly, air 5 is abruptly blown out to the air-blowing means 4. Furthermore, if there is a periodic fluctuation in the pressure of the air 5, there is also an accompanying periodic fluctuation in the back tension that is applied to the wire 1. In other words, the wire 1 is caused to swing violently by the air 5 that is abruptly blown out and also caused to swing periodically by the periodic fluctuation in the back tension. As a result, the wire 1 is beaten against the air guide plate 6 and wire guide 7 or is caused to rub against these elements, so that the wire is bent and the surface of the wire I is scratched and contaminated. This phenomenon becomes more conspicuous for wires having a smaller diameter.